This will either reduce your potential losses, increase your potential winnings or a combination of both. It will never increase your potential losses. Your bet will never become matched at less favourable odds than you requested.

Lay example

A £0.50 lay bet is placed on odds of 5.0, but matched at 4.0. The original bet quantity is £2.50 (0.50*5). Therefore the new stake will be £2.50/4, which is £0.625, while the liability changes from £2 to £1.875.

Back example

A £5.00 back bet is placed on odds of 3.0, but matched at 4.0. The original bet quantity is £15.00 (5*3). Therefore the new stake will be £15/4, which is £3.75, while the potential winnings improves from £10 to £11.25.